Electronically controlled ER fluid dampers have been previously known, but have not been used in semi-active vehicle suspension systems to control the ride and handling of the vehicle. ER fluid dampers are well-suited for use in semi-active suspension systems because their fast response times enable them to be rapidly adjusted in response to varying conditions. A common control algorithm for such dampers, known as the "Sky-Hook" model, typically involves a feedback loop that adjusts damping level based on the vertical velocity of the sprung body of the vehicle and the velocity of the damper's piston.
Heretofore the "Sky-Hook" model has been implemented using conventional logic control, including "on/off", continuous, "soft-soft" and "hard-hard". To control the damping force of the ER fluid damper, for example, threshold values would be predetermined for the detected velocities which, if exceeded, would cause the mode of the damper to be changed from "soft" to "hard". Tuning of these conventionally controlled suspension systems to suit a particular application, however, was a time consuming task sometimes taking many weeks if not months to perform. The procedure typically involved several technicians who ride in the vehicle and subjectively evaluate its ride and handling for comfort, safety and performance. Adjustments would be made and the vehicle would again be tested. This process would be repeated until the desired compromise between ride and handling was achieved.
Attempts have been made to apply fuzzy logic theory to the control of adjustable dampers in a vehicle. Unlike conventional logic wherein a condition is met or not met, fuzzy logic takes into account the degree by which the condition is or is not met. Instead of a road condition being smooth or not smooth (rough), a fuzzy variable state may be very smooth, slightly smooth, slightly rough or very rough. Known prior attempts have used fuzzy inference based on the detected running condition of the vehicle and, more particularly, detected steering angle, vehicle speed, vertical acceleration of the vehicle, braking action and throttle opening speed to control damping force. However, none of these known attempts have used the vertical velocity of the sprung body of the vehicle and the relative velocity between the sprung body and unsprung mass as fuzzy variable inputs nor has there been any recognition in the art of benefits afforded thereby such as ease in tuning an ER fluid damper in a vehicle suspension system.